Oak Switchplate Covers

I have several light switch cover plates that are bowing out. There doesn’t seem to be any pressure on them as if I overtightened the screws. It seems like heat or something else is causing them to bend out. I did find a way to straighten them though… Place them in the oven at 225 degrees for about 10 minutes or so. Flat now but why did they bend in the first place…..

Is my only option to chip away at the pain around the edges and screws? Any ideas?

where is a good place to buy light switch plates? i know lowes and stuff…but i want cute ones! any good store suggestions? thanks